Various forms of penis prosthesis have been developed, for example comprising an implantable rod member of plastic material which can be bent into the respectively desired shape by virtue of the provision of an insert of fine silver wires in the form of a braided component, as disclosed for example in DE-U-83 35133.7. Another form of penis prosthesis comprises an implantable rod member which is made from plastic material and which includes an erection chamber that can be filled with a fluid under pressure, and a hose connection communicating with the erection chamber. In the rest condition, such a prosthesis is bendable and flaccid. When the erection chamber container contained in the prothesis, in the form of a tubular cavity, is put under pressure by the introduction of a fluid, for example by means of a small pump which is also implanted, the prosthesis expands and at the same time becomes stiff. The prosthesis is returned to its rest condition by releasing the pressure from the returned to its rest condition by releasing the pressure from the erection chamber, by the actuation of a suitable valve.